This invention relates to emission computed tomography and, more particularly, to emission computed tomography wherein .gamma.-rays emitted from radioisotopes, distributed in a patient's body, are detected from a plurality of directions around the subject, and a radioisotope density distribution image on any section of the subject is reconstructed on the basis of the data obtained from the radioisotopes.
Conventional emission computed tomography, as shown in FIG. 1, includes a .gamma.-ray detector 1 which rotates around a subject 2, ordinarily a patient's body, to detect .gamma.-rays emitted from a plurality of directions so as to obtain projection images. In this case, the detector 1 is caused to move in a circular orbit 3 regardless of the sectional shape of the subject which, in this case for purposes of illustration, is substantially that of an ellipse. In the conventional apparatus, if the sectional shape of the subject is substantially an ellipse, the detector 1 in a circular orbit would be gradually withdrawn from the surface of the subject as the detector moves from the long axis to the short axis direction of the ellipse and approaches the surface of the subject from the short axis to the long axis direction of the ellipse. Therefore, since the detecting characteristics of the detector 1 tend to change with the rotating positions of the detector, it is impossible to obtain a projection image of well proportioned resolution. Additionally, the images obtained may be of inferior resolution since, when the detector 1 is in a position which is far from the surface of subject, the tomographic image reconstructed on the basis of the images will be of lower quality.